The forklift has attachments such as a mast and a fork as working machines at a front portion of the vehicle body. By operating a working machine operating lever, the working machines are driven to tilt the mast or to lift the fork as an attachment, and the position and posture of the cargo placed on the fork can be changed to a desired position and posture.
The forklift has a vehicle weight ratio of about 1.8 times between an unloaded time and a maximum loaded time. Thus, the vehicle weight ratio is very large.
But, conventionally, engine characteristics were determined in conformity with the maximum loaded time and a maximum travel load time.
FIG. 1 is a torque curve diagram showing engine output characteristics of a conventional forklift. The horizontal axis indicates an engine speed N, and the vertical axis indicates an engine torque T. L is a maximum torque curve of the engine and fixed to one type. The engine speed N corresponds to an acceleration opening degree.
The maximum torque curve L is set so that the forklift can secure a maximum lifting speed under a maximum imposed load and can secure maximum hill-climbing ability under a maximum load (maximum imposed load and maximum travel load).
Regulation lines LL . . . Li . . . LH are determined for respective engine speeds, namely for respective acceleration opening degrees, and when the acceleration opening degree (engine speed Ni) is determined, a matching point between an engine absorption torque and a load moves along a corresponding regulation line Li while the engine speed N decreases depending on the magnitudes of the load. At a certain engine speed Ni, a matching point is positioned at a point having a maximum torque on the regulation line Li, namely a point P on the maximum torque curve L. Control that increases the engine output torque T while decreasing the engine speed N is realized by mechanical control or electronic control according to an all speed control type governor.
The following Patent Reference 1 discloses an invention that aims to control automatically the power output capacity of the engine depending on a working state of a wheel loader and to select automatically a maximum torque curve of the engine by detecting the working state on the basis of a pressure of a hydraulic cylinder of an arm of the working machine of the wheel loader and an inclinometer of the working machine of the wheel loader.
The following Patent Reference 2 discloses an invention that aims to obtain acceleration characteristics required for a forklift having a battery as a drive source depending on a use situation and sets variably an initial current value supplied to a running motor depending on the weight of the cargo loaded on the forks.
The following Patent Reference 3 discloses an invention in which a forklift is provided with a load measuring device for measuring the load of a cargo placed on its forks, the load measured by the load measuring device is shown on a display, an alarm is issued if the measured load exceeds a predetermined value, and a forward inclination angle of the mast and a travel speed are limited depending on the measured load.    Patent Reference 1: WO2005/024208 A1    Patent Reference 2: Japanese Patent Application Laid-Open No. 2003-54899    Patent Reference 3: Japanese Patent Application Laid-Open No. 2010-6604